1. Field of Invention
The present invention relates to the field of shaving by mechanical means using a discardable razor; it relates more particularly to an auxiliary shaving article in the form of a strip and referred to as a xe2x80x9cstripxe2x80x9d, which is fixed on the shaving head, preferably in the vicinity of the shaving blades, and which contains at least one shaving-assistance component that is soluble in water.
2. Description of the Related Art
When the razor is used, the user dips the shaving head in hot water, thereby making a portion of the shaving-assistance components) accessible and leachable, which components come into contact with the skin on each pass of the razor.
In U.S. Pat. No. 4,170,821 to Booth, the auxiliary shaving article proposed is combined with a micro-encapsulating or micro-porous water-soluble solid structure for retaining the shaving-assistance component which can be selected from a list of substances comprising a lubricant for reducing the effect of friction between the razor and the skin, an agent for reducing rubbing between the razor and the skin, an agent suitable for modifying hair structure, a cleansing agent, a pharmaceutical agent, a cosmetic agent, and a coagulation agent. As an example of a lubricant for reducing the effect of friction between the razor and the skin, U.S. Pat. No. 4,170,821 cites a micro-encapsulated silicone-based oil, and as an example of an agent for reducing rubbing between the razor and the skin, that document cites an oxide of polyethylene having molecular weight lying in the range 100,000 to 6,000,000, a non-ionic polyacrylamide, and a natural polysaccharide derived from plant matter such as guar gum.
In document EP-B-0 1 184 440, provision is made to form an auxiliary shaving article in the form of a strip by extruding a mixture of water soluble and insoluble polymer materials. That document gives a list of water soluble polymers comprising polyethylene oxide, poly-vinylpyrrolidone, polyacrylamides, hydroxypropyl-cellulose, polyvinylimidazoline, and hydroxyethyl poly-methacrylate. In a particular embodiment, the mixture for forming said strip by extrusion is made up of 20% by weight water insoluble polystyrene and 80% by weight of a mixture of polyethylene oxide comprising 60% xe2x80x9ccoagulating polyoxxe2x80x9d polyethylene oxide of molecular mass equal to 5,000,000, and 40% xe2x80x9cWSR N-750 polyoxxe2x80x9d polyethylene oxide of molecular mass equal to 300,000. No explanation is given in that document concerning the advantage of mixing those two types of polyethylene oxide together, other than for obtaining a mean molecular weight of about 3.5 million for the polyethylene oxide in the final mixture.
In document EP-B-0 550 605, the mixture for forming the auxiliary shaving article by extrusion in the form of a strip contains, in addition to the water soluble and insoluble materials, an agent of low molecular weight for amplifying the release of the water soluble polymer material making up the shaving-assistance component. Amongst all of the possible examples, that document cites polyethylene oxide as a shaving-assistance component which can be leached by water, and polyethylene glycol as an agent for amplifying release. In characteristic manner, according to that document EP-B-0 550 605, the mixture for forming the auxiliary shaving article by extrusion in the form of a strip comprises 20% to 60% by weight of insoluble polymer material that forms the matrix of the strip, 20% to 75% by weight of water soluble polymer material that constitutes the water-leachable shaving-assistance component, and 5% to 20% by weight of agent for amplifying release. In all of the examples cited in that document, exactly the same mixture of two types of polyethylene oxide is to be found as already described in prior document EP-B-0 184 440, i.e. 60% by weight of coagulating polyox and 40% by weight of WSR N-750 polyox. In the examples cited, the agent for amplifying release is a polyethylene glycol of molecular weight lying in the range 4,500 to 20,000. The intended purpose of having the release-amplifying agent present is to make it possible to maintain a sufficient quantity of insoluble polymer for retaining sufficient mechanical strength in the extruded strip both on initial manufacture and assembly, and after a significant quantity of water soluble material has already been leached, while still making it possible for a sufficient quantity of water soluble shaving component to be released to provide effective shaving assistance throughout the total useful lifetime intended for the blade or blades.
In that document EP-B-0 550 605, no explanation is given concerning the reasons which enable certain specified agents of low molecular weight to amplify release of the shaving-assistance component, i.e. ethylene glycol, methoxy polyethylene glycol, methyl-cellulose, and carboxypolymethylene. It should be observed that those four examples are the only examples mentioned in that document, and that they all relate to the above-mentioned mixture of polyethylene oxide and the added polyethylene glycol. It should be recalled that polyethylene oxide and polyethylene glycol have the same general formula, differing only in method of manufacture and mean molecular weight. The term xe2x80x9cpolyethylene glycolxe2x80x9d is used to designate a compound whose molecular weight is generally less than 20,000. The term xe2x80x9cpolyethylene oxidexe2x80x9d is used to designate a compound whose main molecular weight is greater than 100,000, it being understood that a very wide variety of products are available on the market, having the general formula of polyethylene oxide, with mean molecular weights lying in the range 100,000 to 8,000,000.
Thus, according to the Applicant, it can be thought that polyethylene glycol is used in document EP B 0 550 605 also as a shaving-assistance component, in addition to the low molecular weight polyethylene oxide (WSR N-750 polyox) so as to obtain sufficient release of shaving-assistance components during the lifetime of the discardable shaving head.
Thus, present trends in this field are towards releasing a large amount of shaving-assistance soluble component, in particular polyethylene oxide which is preferably associated with polyethylene glycol. Nevertheless, because a large amount is released, the component remains present on surfaces of the skin that have already been shaved, and can form a film by drying out. To avoid that drawback, it is necessary for the user to wash after shaving. However, depending on the quality of the water available, this removal is not always satisfactory and a sticky feeling can remain on the skin. If all or some of the soluble component remains on the skin, that can be a source of irritation, particularly for sensitive skins.
To mitigate those drawbacks, proposals have already been made in document EP-B-0 321 679, for a solution that is different in principle since it avoids the use of a soluble polymer by proposing to implement a xerophilic gel as the anti-friction agent which, while absorbing water as a dispersing agent, becomes transformed into a lyophilic gel having very great aptitude for sliding on the skin of the user, with a coefficient of friction xcexc of less than 0.25. By adding the dispersion agent, e.g. water, the xerophilic gel becomes transformed once more into a lyophilic gel by swelling, with its outside surface becoming slippery and presenting a low coefficient of friction. During this stage, the colloidal substance forming the-lattice of the lyophilic gel does not pass into solution, thereby making it possible to avoid forming on the skin a film constituted by a shaving-assistance component extracted from the strip, as was the case in the previously-cited documents.
Nevertheless, according to the teaching of that document EP-B-0 321 679, it is necessary to put the coating that forms the xerophilic gel on a support strip in order to constitute the anti-friction strip proper for placing on the shaving head. The materials recommended for forming the xerophilic gel do not withstand temperatures that would make it possible to consider fabricating the anti-friction strip by extrusion.
The object of the Applicant is to propose another solution that does not make use of a xerophilic gel, that makes it possible to obtain an anti-friction strip by extrusion, while enhancing ability to slide on the skin of the user, and without running the risk of forming a film of the soluble component on the user""s skin.
This object is achieved in full by the anti-friction strip for discardable razors of the invention which, in conventional manner, is obtained by extruding a polymer mixture of an insoluble polymer material and of polyethylene oxide as the soluble polymer material. In manner characteristic of the invention, said strip is characterized by an initial coefficient of dynamic friction (KD) of the order of or less than 0.2, and by the fact that the mixture to be extruded comprises, as its polyethylene oxide, only a substance whose mean molecular weight is greater than 3.5 million.
The Applicant has observed that the presence of polyethylene oxide of lower molecular weight, even when mixed with polyethylene oxide of higher molecular weight, leads to an increase in the coefficient of dynamic friction.
It also turns out that the greater the molecular weight of the polyethylene oxide, the smaller its solubility in water. Thus, with a polyethylene oxide of molecular weight greater than 3.5 million, release is obtained that is relatively low compared with the figures given in documents EP-B-0 184 440 and EP-B-0 550 605. The risk of skin irritation or of the sensation of slippery skin is thus greatly reduced or even eliminated by this fact.
During comparative shaving tests, the Applicant has also observed another significant technical advantage in using only a polyethylene oxide of high molecular weight in the mixture, namely that of retaining sliding capacity for the anti-friction strip which remains substantially constant or which varies relatively little during a series of shaves using the same strip. By way of comparison, an anti-friction strip including polyethylene oxide of lower molecular weight presents a significant drop in its sliding capacity after two or three shaves, thereby giving rise to a sensation that is quite disagreeable for the user.
Preferably, with reference to this technical advantage, the anti-friction strip is obtained by extruding a mixture of polystyrene and a polyethylene oxide of mean molecular weight that is greater than 4 million.
Another problem which the Applicant has sought to resolve lies in fixing the anti-friction strip to the shaving head. In document FR-A-2 410 541, the strip is fixed to the shaving head, preferably in a housing provided for that purpose. No indication is given as to how it is fixed therein. In document EP-B-0 184 440, the strip has a special profile, and it is locked in position by structural elements on the shaving head. That solution makes fabrication of the head more complex. In document EP-B-0 550 605, the anti-friction strip is fixed by means of adhesive in a hollow portion of the shaving head. That solution requires the use of an adhesive that sets particularly quickly if a high rate of manufacturing throughput is to be obtained.
In order to fix the anti-friction strip on the shaving head, the Applicant proposes the well-known technique of ultrasonic welding. This technique consists in creating vibratory conditions under the action of ultrasound that make it possible, locally, to raise the temperature of two touching plastics materials so as to obtain localized interpenetration of the two materials.
The Applicant has observed that by using the ultrasonic welding technique with an anti-friction strip of polyethylene oxide having a molecular weight greater than 3.5 million makes it possible to obtain a weld that is of good quality. A weld between a first plastics material acting as a base and a plastics material applied thereto is said to be of xe2x80x9cgood qualityxe2x80x9d when breaking the weld by applying mechanical stress gives rise to breakage taking place for the most part in the base material or in the material applied thereto, but not within the weld proper. Material is, so to speak, torn beyond the zone in which the two materials have interpenetrated.
Best results have been obtained with an anti-friction strip made by extruding a mixture of poly-ethylene and a polyethylene oxide having a mean molecular weight in excess of 7 million, which is fixed by ultrasonic welding onto a portion of a shaving head that is made of polystyrene.